The Gram-negative bacterium Burkholderia pseudomallei is a soil saprotroph and an emerging infectious disease agent. It is the causative agent of melioidosis, which can result in acute pulmonary disease, septicemia, and can be fatal if left untreated. Although naturally found in Southeast Asia and Northern Australia, it is of globl concern because of its potential to be weaponized. Bacteria typically live in polymicrobial communities and hence have evolved mechanisms to communicate, cooperate and compete with their neighbors. One such mechanism is the contact-dependent growth inhibition (CDI) system. This system is composed of two-partner secretion proteins, and is utilized by the bacterium to kill neighboring cells and potentially control the surrounding population. The main component of this system is a large surface exoprotein, BcpA, which contains a toxic C-terminus (CT). The CT is delivered to a target bacterial cell to induce death, unless the target cell contains an immunity protein to block the toxic activity. A hallmark feature of CDI systems is polymorphism among CTs and their cognate immunity proteins that protect in an allele specific manner. Studies in our laboratory have shown that the CDI system produced by the non-pathogen B. thailandensis is used for interbacterial competition and biofilm formation. In this proposal we will address several questions regarding Burkholderia-type CDI systems. First we will assess the ability of B. pseudomallei to utilize its CDI systems for interbacterial killing. W will then establish a role for CDI systems in interspecies killing when the bacterium comes into contact with closely related organisms. To achieve this we will evaluate interspecies competition between B. pseudomallei and B. thailandensis. Additionally, through studies using BcpA chimeric proteins, we will analyze regions of BcpA that potentially play a role in activity and specificity of the system. Lastly, we will perform screening analyses to identify the proteins on the target cell that are recognized by the toxin. This step is essential to promote cell-to-cell contact and it defines the specificity of the system. The different aspects that we will investigat in this study will contribute to the understanding of the role of CDI systems within a microbial community. This knowledge can be applied towards biofilm communities that are present in the soil, which is the source of transmission of B. pseudomallei to humans.